In an industrial installation a distributed control system (DCS) or an industrial control system may be equipped with a large number of field devices, including sensors and instruments. Usually such an industrial installation has one or more control rooms and a computer system which are connected by one or more data networks, such as buses or fieldbuses to the various field devices. The data network may comprise both wired and wireless links. Field devices may include various types of sensor instrument such as vibration sensors, pressure sensors, temperature gauges, as well as devices that include an actuator such as a valve actuator, a solenoid, a breaker or isolator or on/off switch and the like. Each field device such as a wireless sensor needs to be configured suitably to carry out the chosen function of monitoring an individual device. Each wireless sensor and field device at a specific location are usually given an identity in the control system or DCS. A wireless sensor may also have an identity in a wireless network.
Typically Wireless Sensor Networks (WSN) have been used to collect a couple of bytes (e.g. a temperature value) from each node. Vibration is, however, a more demanding phenomenon to monitor with wireless communication because of the large amount of data (e.g. 4 or 8 kB of data) that is needed for the vibration analysis. An important technical requirement for a wireless sensor is that of power use. For sensors which are not supplied from the grid or another power supply, but supplied instead by an independent power source such as a battery or a fuel cell, the power characteristics of the power source and useful lifetime between replacement are critical factors. The power use must be sufficiently low so that a battery will last for a period of 1 or more years between replacement.
U.S. Pat. No. 7,424,403 entitled Low power vibration sensor and wireless transmitter system sensor and wireless transmitter system, assigned to CSI Technology Inc., describes a sensor system that senses parameters of a machine including vibration and produces dynamic signals representing the sensed parameters. The system converts the signals to a digital format, digitally filters and processes the signals. A processor 20, preferably a digital signal processor DSP, determines a plurality of levels relative a predetermined set of data points of the digital signal. The processor determines a peak level value and calculates a true root-mean-square. The peak level and the RMS value are communicated wirelessly by a communication module of the system to a control protocol network. A system power supply powers both the processor and a communication module. The power supply from the battery to the measurement system, not including the communication module, is controlled by the processor 20. However, a disadvantage of this approach is that analysing measurements in the processor of the sensor before wirelessly transmitting such processed data requires additional memory storage, some delays, more processing power and uses more electrical power from the battery. This is often the case when a manual request for a data sample is made by an operator who waits for the processed result.
In an environment such as an installation for extraction, production or processing of oil, petrochemicals and/or gas it is vital that the equipment complies with standards to avoid explosions or fires should explosive vapours be present. For example a European ATEX standard (Appareils destinés à être utilisés en ATmosphères EXplosibles) compatible with equipment directive 94/9/EC regulates aspects such as avoiding that an equipment can become a source of ignition. Thus all equipment is required to be fully enclosed so that gases and etc do not normally enter the equipment. The ATEX requirement also involves avoiding that components can become sufficiently hot to ignite explosive gas or vapour, both during normal operation and in a possible failure mode. For example, if a battery should fail with an internal short circuit, that failure should not be allowed to lead to an ignition source. Thus there is a technically challenging simultaneous requirement for stand alone wireless sensors with an energy source with sufficient energy density to power a sensor for a long period of time versus the requirement to avoid that any component can become hot or overheated during use or failure.